The present invention relates to an apparatus for measuring small displacements.
It has been proposed to measure small displacements accurately using sensors sensitive to magnetic fields and changes in magnetic fields. The effect referred to is not of voltage generation due to a conductor cutting magnetic lines, but is a static effect of a magnetic field on a sensor which is affected thereby. Among sensors useful for this are so-called `barber pole` elements, and Hall pressure sensors. Relative movement between a sensor and a magnet can effect a change in electrical output, and an array of sensors in line or in two dimensions gives accurate data concerning the position of the movable element. The most practical arrangement uses an array of barber pole elements at discrete spacings, with a magnet movable relative thereto. A practical use is with a pressure sensor having a deflectable membrane, a permanent magnet being mounted on the latter and a row of elements mounted on the casing. Apparatus of this general type has the disadvantage that since the voltage output of each barber pole element is at a discrete level, in the general case when the magnet lies between two elements, an interpolation must be carried out to decide the exact location of the magnet. Only if the magnet were directly above a barber pole element would this give a zero output, with consequent certainty as to the position of the magnet. In the past, the interpolation has been made using separate microcomputer facilities, which is expensive, and is even then only an approximation.